This study will explore whether the combination of nelfinavir (NFV) plus DMP-266 in combination with nucleoside analogue inhibitors of HIV-1 reverse transcriptase (RT) will be more effective in suppressing HIV-1 replication than nucleoside RT inhibitors plus either NFV or DMP-266 alone, in nucleoside-experienced patients. This is a randomized, partially double-blind trial for ACTG 302/303 participants who were originally randomized to d4T monotherapy or new combination nucleoside therapy and who remain on their originally assigned treatment at the time of enrollment into this rollover protocol. It is estimated that 300 subjects will be eligible. Subjects will be stratified on the number of new nucleoside analogs they will receive (1 or 2), relative to the combination nucleoside therapy they received in ACTG 302/303. This study stratifies subjects into 1 of 2 Treatment Groups (Treatment Group A vs. Treatment Group B) based upon level of HIV RNA copies/ml at baseline and allows crossover during study if HIV RNA levels change from undetectable to detectable status.